Long-term behavior of cloud systems in TOGA COARE and their interactions with radiative and surface processes. Part I : Two-dimensional modeling study

Two-dimensional cloud-resolving modeling of tropical cloud systems was performed for a 39-day period (5 December 1992 through 12 January 1993) during the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean-Atmosphere Response Experiment (COARE). This period contains strong convective episodes within a major westerly wind burst. A cloud-resolving model combined with a cloud-interactive radiation scheme was driven by the observed evolving large-scale forcing for temperature and moisture, evolving large-scale horizontal winds, and evolving sea surface temperature. These all represent averaged conditions over the Intensive Flux Array of TOGA COARE. Model-produced quantities were evaluated against the observational data and used to quantify interactions of the simulated cloud systems with radiative and surface processes. Focus is on quantities of value to cloud-climate research and to an improved physical basis for the parameterization of cloud systems per se. During convectively disturbed periods (strong forcing), the collective effects of cloud systems on the thermodynamic fields and budgets, radiative fluxes, surface heat fluxes, and surface energy budgets are well simulated. However, during relatively undisturbed periods (weak forcing), the comparison between model-produced quantities and observations is less satisfactory. Specifically, the domain-averaged temperature and moisture fields of the model are too warm and too moist, and the domain-averaged outgoing longwave radiative flux is too small. The absence of observed data to specify large-scale forcing for the condensed water, which is required by the use of periodic lateral boundary conditions, is argued to be one of the reasons why the cloud (ice and liquid) water content is too large. The role of cloud systems and oceanic processes in the regulation of the sea surface temperature over the tropical oceans is also discussed.